1. Field of the Invention
The present invention relates to wind noise reduction devices and wind noise reduction methods for reducing wind noise contained in an input sound signal, and also relates to sound-recording apparatuses, image-sensing apparatuses, and electronic appliances employing such wind noise reduction devices.
2. Description of Related Art
In a sound-recording apparatus equipped with a microphone, when the microphone is exposed to wind, the sound signal is corrupted with wind noise. The wind noise results from the pressure of wind striking the diaphragm of the microphone. Not intrinsic in the sound signal, the wind noise should ideally be eliminated.
To prevent wind noise in outdoor sound recording, it is common to fit the sound-collecting device, such as a microphone, with a wind-shielding device, such as one called “Germer”, or to cover the sound-collecting device with urethane. Inconveniently, however, in compact electronic appliances, such as compact video cameras, furnished with sound-recording capability, seeking the compactness of the appliances themselves makes it difficult to fit their integrated microphone with a mechanical wind-shielding device. These appliances thus incorporate, instead of a mechanical wind-shielding device, a wind noise reduction device.
Wind noise lies in a relatively low frequency band, typically concentrating in a band of about 300 Hz and below. This characteristic is exploited by the conventional wind noise reduction device, which reduces wind noise in, mainly, a low-band signal. The typically used method is to split, by use of a high-pass filter (HPF) and a low-pass filter (LPF), the input sound signal into low-band components and higher-band components, then reduce (or eliminate) the low-band signal, and then add the low-band and higher-band components together again.
Some conventionally proposed wind noise reduction devices are additionally provided with a function for checking the presence of wind noise. The check for the presence of wind noise typically exploits the characteristic of wind noise that “wind noise does not exhibit cross-correlation between the left- and right-channel signals composing an input sound signal”. Specifically, the cross-correlation between the left- and right-channel signals composing an input sound signal is found and, if the correlation value that indicates the cross-correlation is equal to or smaller than a given threshold value, it is judged that the input sound signal contains wind noise. The correlation value thus found is used not only to check the presence of wind noise but also as an index representing the intensity of the wind noise. For example, there have also been proposed methods that vary, according to the correlation value, the degree to which the low-band signal is reduced.
The low band includes the frequency band of wind noise, and is much affected by wind noise; in addition it also includes the essential elements of sound. In particular, the pitch of the human voice (more precisely the fundamental frequency of that pitch) ranges from about 90 to 160 Hz in males and from about 230 to 370 Hz in females, and thus the essential elements of the human voice, determining its timbre (quality), lie in the low band. The pitch here denotes the fundamental frequency and harmonic components of a signal resulting from the vibration of the vocal cord. If the components in this band including those essential elements are simply reduced or eliminated, even the elements of signal components other than those of wind noise are reduced or eliminated, leading to distorted sound—in the case of the human voice, its volume diminishes and its timbre changes.
Moreover, if wind noise reduction is applied only in the low band and not in the other band, wind noise of relatively high frequencies remains (heard as a sound like something rolling), causing the user to hear unnatural sound.
The configuration of another conventional wind noise reduction device is shown in FIG. 22. The wind noise reduction device of FIG. 22 has largely the same configuration as that of FIG. 11. The wind noise reduction device of FIG. 22 too exploits the characteristics of wind noise that it concentrates in a low band and that it does not exhibit cross-correlation between the left- and right-channel signals. The sound signals from a microphone that collects sound from the left and right sides independently (hereinafter “stereo microphone”) are fed to the wind noise reduction device of FIG. 22. The sound signals representing the sound collected by the stereo microphone from the left and right sides are called the L and R signals respectively.
The wind noise reduction device shown in FIG. 22 comprises: a correlation-value calculator 201 that calculates the correlation value between the L and R signals output from the stereo microphone; low-pass filters (LPFs) 202L and 202R that pass the low-band components of the L and R signals respectively; high-pass filters (HPFs) 203L and 203R that pass the high-band components of the L and R signals respectively; attenuation circuits (reduction circuits) 204L and 204R that attenuate (reduce) the low-band components that have passed through the LPFs 202L and 202R respectively; and addition circuit 205L and 205R that add the low-band components from the attenuation circuits 204L and 204R to the high-band components that have passed through the HPFs 203L and 203R respectively.
In the wind noise reduction device configured as described above, the correlation-value calculator 201 calculates the correlation value between the L and R signals, and thereby sets the amount of signal attenuation effected by the attenuation circuits 204L and 204R. Specifically, when the correlation value calculated by the correlation-value calculator 201 is smaller than a predetermined threshold value, it is judged that the signals contain wind noise, and the amount of attenuation effected by the attenuation circuits 204L and 204R is increased. By contrast, when the correlation value calculated by the correlation-value calculator 201 is larger than a predetermined threshold value, it is judged that the signals do not contain wind noise. In this case the attenuation circuits 204L and 204R do not effect signal attenuation (reduction); thus the low-band components that have passed through the LPFs 202L and 202R are, intact, fed to the addition circuit 205L and 205R.
The LPFs 202L and 202R have such a filter characteristic as to pass low-band components down to several kHz, and the HPFs 203L and 203R have such a filter characteristic as to pass high-band components that cannot pass through the LPFs 202L and 202R. Thus the low-band components that pass through the LPFs 202L and 202R contain almost all wind noise components that can be contained in the sound signals. The attenuation circuits 204L and 204R attenuate (reduce) these low-band components, and thus the L and R signals output from the addition circuit 205L and 205R contain almost no wind noise components.
In the conventional wind noise reduction device as exemplified by that of FIG. 22, the cut-off frequencies of the LPFs and HPFs are fixed, and thus wind noise is reduced only in the frequency band in which the LPF pass. In reality, however, a strong wind may produce wind noise in a band beyond the cut-off frequency of the LPFs, in which case the conventional wind noise reduction device cannot satisfactorily reduce the wind noise. For example, when sound signals containing wind noise in a band ranging from DC (direct current) to a frequency Fx as shown in FIG. 23A are fed to the conventional wind noise reduction device, if the cut-off frequency of the LPFs equals fc lower than the frequency Fx, then, as shown in FIG. 23B, the wind noise in the band between the frequencies fc and Fx is not reduced. As a result, wind noise of relatively high frequencies remains (heard as a sound like something rolling).
There has also been proposed a technology that employs a wind pressure sensor disposed beside a microphone to set, according to the wind pressure signal output from the wind pressure sensor, the cut-off frequency below which to cut off low-band components. Inconveniently, however, the additional provision of the wind pressure sensor hampers miniaturization of apparatuses.